1. Field of the Invention
This invention relates generally to the field of semiconductor processing and more particularly to a method of fabricating a photolithography mask and pellicle.
2. Description of the Related Art
The fabrication of microelectronic integrated circuitry generally involves the patterning of device structures and layouts on a semiconductor substrate. The accepted practice for creating the requisite pattern is to first form a replica of the pattern on a mask (not necessarily in its final size) and then to transfer the mask pattern to a layer of photoresistive material formed on the semiconductor substrate. The transfer is accomplished by a photolithographic process, shining light of a certain wavelength through the mask and onto the photoresist, using whatever lenses are required to replicate the pattern in its proper size on the photoresist. Once the pattern has been transferred to the photoresist, the photoresist is processed to selectively remove portions of the pattern and expose the substrate below. The substrate, itself, can then be etched by, for example, an anisotropic plasma etch, or otherwise processed as required.
As the dimensions of pattern features become increasingly small, more stringent requirements are placed on the formation of the mask and on methods for preventing mask damage both during fabrication and during use. The process by which the mask is formed is itself quite similar to the process in which the mask is used to pattern the semiconductor substrate. Typically, the mask is formed on a substrate that is transparent to the range of light wavelengths that will be used to pattern the substrate. A layer of opaque material, typically chromium, is formed on the transparent substrate. The opaque layer is then covered by a photoresistive layer and the pattern is traced out in this layer by electron beam lithography. Finally, the photoresist is selectively removed and the opaque layer beneath it is etched away by either isotropic wet etching or anisotropic plasma etching.
It is quite important that the mask so formed be inspected for defects and that those defects be either removed or repaired. This process is implemented by either laser ablation or by the application of a focused ion beam (FIB). Laser ablation, however, can damage the underlying substrate and the Gallium ions used in FIB can stain and dope the substrate.
The completed mask is typically covered by a protective material, called a pellicle. The purpose of the pellicle is to prevent foreign particles from contaminating the mask surface. As shown in FIG. 1, the pellicle is typically a membrane (10) of an organic material that is stretched over the mask (40) on a plastic or metal frame (30) and is raised several millimeters from the mask surface. The pellicle limits the ability to optically (20) inspect the surface of the mask prior to usage. In addition, the pellicle traps gases (50) that can limit the transmission of small wavelength electromagnetic radiation.
Kubota et al. (U.S. Pat. No. 5,370,951) teach a method of forming an improved thin film pellicle that uses a new adhesive to allow the bonding of a group of fluorocarbon membrane materials that have superior resistance to the adverse affects of ultraviolet (UV) radiation in the wavelength range 210-500 nm, commonly used in the photolithography process.
Troccolo (U.S. Pat. No. 5,795,684) teaches a method of forming a photolithographic mask wherein it is the transmissive material that is etched rather than the opaque (absorbing) material and the etched regions are subsequently filled with absorber. The advantages of this method are 1: that the transmissive layer is more robust an permits the use of plasma etching that creates sharply defined etched regions with vertical walls and 2: defects and ion staining can be more easily removed or covered over by the subsequently deposited absorber layer. The method is primarily designed for the fabrication of masks intended for use with light of wavelength down to 193 nm. Light down to 120 nm requires a reflective version of the mask. Following fabrication of the mask, a capping layer, which is a kind of pellicle, can be deposited. This layer is preferably the same material as the transmissive layer and is deposited in a manner to fill all the spaces between the absorbing layer portions.
Scott et al. (U.S. Pat. No. 5,935,733) teaches a method for forming a photolithography mask in which trenches are etched in a transmissive substrate and absorbing material is deposited in those trenches (very much in the manner of Troccolo, above). The fabrication is then planarized and a transmissibe layer is formed on it.
Hibbs et al. (U.S. Pat. No. 5,955,222) teaches a method of forming a rim type phase-shift photolithograph mask suitable for use with radiation in the wavelength range between 190 and 450 nm. A rim-type phase shift mask is one in which there are central openings through which the radiation is transmitted which are surrounded by annular regions containing transmissive material that shifts the phase of the radiation relative to that passing through the central region. When the two rays are combined on the target photoresist, they cancel out possible diffraction patterns that adversely affect the resolution of the pattern transfer process. The method includes the use of a novel xe2x80x9chybridxe2x80x9d photoresist formulation having both positive and negative tone functions depending upon the degree of exposure.
As the size of the smallest features to be transferred by photolithography approach the range of between 70 and 100 nm, the commonly used radiation wavelength of 193 nm becomes too large and a smaller wavelength of 157 nm becomes preferable. Unfortunately, pellicles designed for use with 193 nm radiation are not usable with radiation of 157 nm both because the 157 nm radiation is more strongly absorbed by the pellicle material and because the 157 nm radiation causes radiation damage. For example, air trapped between the pellicle and the mask surface will form ozone which attacks the pellicle membrane material. In addition, Gallium ion stains in the mask substrate produced during mask repair are more troublesome in the context of 157 nm radiation because of reduced transmissivity. It would be advantageous, therefore, to develop a process for forming photolithography masks and their pellicles that would be suitable for use with 157 nm radiation.
A first object of this invention is to provide a method for forming a photolithography mask and pellicle that is suitable for use with incident radiation within a range of wavelengths that extends at least between approximately 250 nm and approximately 150 nm.
A second object of this invention is to provide a method for forming a photolithography mask that can be repaired using standard focused ion beam (FIB) technology without the resultant loss of transmissivity produced in masks of the prior art.
A third object of this invention is to provide a method for forming a photolithography mask pellicle that will not be damaged by radiation within a range of wavelengths that extends at least between approximately 250 nm and approximately 150 nm.
A fourth object of this invention is to provide a method for forming a photolithography mask and pellicle combination wherein said pellicle is a hard pellicle, wherein said mask and pellicle are closely in contact and wherein a bending of the pellicle relative to the mask substrate is avoided.
In accord with the objects of this invention there is provided a method for forming a photolithography mask and pellicle wherein both mask substrate and pellicle are formed of the same transparent hard material, such as fluorine doped silica, which is highly transmissive in the required wavelength range. Further in accord with the objects of the present invention there is provided a method of delineating opaque regions of said mask by patterning and etching a metallic layer such as chromium in a manner as is found in the prior art, but unlike the methods of the prior art said etched metallic layer serves only as a mask for forming the final opaque regions within the transparent material in a completely novel manner. Further in accord with the objects of this invention there is provided a method for forming a photolithography mask wherein the mask repair step is a FIB repair step commonly in use within the practice of the prior art, yet wherein the gallium ion staining, which is disadvantageous in the prior art, is actually used for the process of forming the opaque regions of the mask through the etched metallic regions. Thus, rather than being a detriment to the formation of the mask, the staining process is an advantageous part of its fabrication process. Further in accord with the objects of the present invention and as a part of a second embodiment thereof, there is provided a method of using the FIB repair process to form carbon deposits on the mask for the purpose of forming opaque areas thereon. Yet further in accord with the objects of the present invention, there is provided a method for finally removing those portions of the opaque metallic layer formed upon the transparent mask substrate so that a hard pellicle, formed of the same transparent material of the mask, can be placed against said substrate, aligned with it and be in contact with it, thus eliminating warping of the pellicle and trapping of air beneath the pellicle.